1. Field of the Invention
The present invention relates to a projection-type display apparatus, and more particularly to a technique for realizing a reduction in thickness of a rear-projection-type video apparatus for projecting, in a magnified fashion, a video image, computer-based video picture or the like from behind a screen.
2. Description of Related Art
In recent years, a great variety of video sources having high image quality are becoming widespread, for example, owing to launching of digital television broadcasting service. Also, presentations using a computer-based video picture are becoming commonplace at meetings and conferences. Then, video apparatuses for use for such purposes are increasingly required to have a large-sized screen having high image quality. As a promising technique for realizing such a large-sized screen having high image quality at low cost, there is a rear-projection-type video apparatus. In the rear-projection-type video apparatus, as is well known, illumination light is emitted on a high-intensity CRT (cathode ray tube), a transmission liquid crystal display, a reflection liquid crystal display, a digital micromirror device (DMD) or the like, and image light obtained at the image plane thereof is magnified and projected by a projection optical system. The image light is then projected onto the back side of a screen, so that a viewer can view a video image on the front side of the screen. Therefore, in order to greatly magnify the image light, it is necessary to increase an optical path length thereof correspondingly. The increased optical path length of the image light disadvantageously causes an increase in depth of the apparatus.
In view of this, it is conceivable to project image light obliquely onto a screen so as to decrease the depth of the apparatus, and a variety of proposals based on such conception have been made. For example, U.S. Patent Application Publication No. 2002/0008853 A1 discloses such a projection optical system having much improved performance and indicates that a high-performance video apparatus can be realized by using an oblique projection method.
On the other hand, there is a problem associated with a rear projection screen for use in the oblique projection method as follows. In cases where an ordinary refraction-type Fresnel lens is used in the rear projection screen, the surface reflectance of an entrance surface of the screen is very large due to an increased angle of incidence. Therefore, sufficient brightness cannot be obtained on the screen. In addition, since reflectance rapidly increases as an angle of incidence becomes larger, uniform brightness cannot be obtained. To overcome such problems, there is a proposal for the usage of a total-reflection-prism-type Fresnel lens as disclosed in, for example, U.S. Pat. No. 4,674,836. In the total-reflection-prism-type Fresnel lens, a number of prisms are arranged along arc lines and total internal reflection occurs within each prism, so that sufficient brightness can be obtained even if an angle of incidence is large.
Folding an optical path by using a plane mirror is effective for reducing the depth and height of the apparatus in the oblique projection method. For that purpose, there have been made a number of proposals. For example, the above-mentioned U.S. Patent Application Publication No. 2002/0008853 A1 discloses using a mirror that is approximately parallel to a screen. Also, the above-mentioned U.S. Pat. No. 4,674,836 discloses using a mirror that is approximately perpendicular to a screen.
Using both the projection optical system and the total-reflection-prism-type Fresnel lens disclosed in the above proposals may make it possible to realize a rear-projection-type video apparatus having a reduced depth capable of obtaining uniform brightness. However, there are some problems in practice. For example, in the total-reflection-prism-type Fresnel lens, if an angle of incidence is relatively small, some incident rays may pass thorough the Fresnel lens without impinging on a total-reflection surface, as shown in FIG. 11. Therefore, it is necessary to make an apex angle θt of each prism sufficiently small.
In FIG. 11, a ray “b” which falls on a screen 102 at an angle α relative to the normal 121 to the screen 102 is incident on an entrance surface 181 of a prism PR101 at an incident angle θi. The ray “b” is refracted at the entrance surface 181 according to Snell's law of refraction and becomes a refracted ray “b′”. The refracted ray “b′” emerges from the entrance surface 181 at an exit angle θr and is incident on a total-reflection surface 182 of the prism PR101 at an angle larger than a critical angle. Then, the ray “b′” is reflected and bent 100% by the total-reflection surface 182 and becomes a ray “c” that is approximately perpendicular to the screen 102. The apex angle θt of the prism PR101 and an angle θs of the entrance surface 181 relative to the surface of the screen 102 are so predetermined as to ensure these actions of the total-reflection-prism-type Fresnel lens. However, a ray “b2” nearer the tip of an adjoining prism PR102 than the incident ray “b” advances straight as a ray “b2′” without impinging on the total-reflection surface 182. The straight advancing ray “b2′” not only results in loss in quantity of light but also becomes the cause of a phenomenon in which an image appears in a position different from an original position, what is called a ghost image, thereby remarkably deteriorating video image quality.
Reducing the apex angle θt may contribute to preventing such a phenomenon, but causes another problem. FIG. 12 illustrates a case where an apex angle θt of each prism is reduced with respect to the same incident angle α as in FIG. 11. In FIG. 12, the same or similar parts as in FIG. 11 are denoted by like reference characters. An incident ray “b” is bent likewise and becomes a ray “c” that is approximately perpendicular to the screen 102. Because of the apex angle θt being reduced, a ray “b2” passing near the tip of the adjoining prism PR102 is also incident on the total-reflection surface 182. The ray “b2” is, therefore, bent in a correct direction and becomes a ray “c2” that is approximately perpendicular to the screen 102. However, a reflected ray from the entrance surface 181 of the prism PR101 may be incident on a total-reflection surface 183 of the adjoining prism PR102 from the back side thereof, thereby becoming astray light to cause a ghost image. More particularly, reflection necessarily occurs at the boundary between media. For example, when a ray passes from air to an acrylic resin having a refractive index of 1.49, about 4% of the ray is reflected. Referring to FIG. 12, reflected light of the incident ray “b” is a ray “d”, which will not return again to the surface of the screen 102. On the other hand, a reflected ray “d2” of the incident ray “b2” enters the adjoining prism PR102 and then advances as a ray “s”, a ray “t” and a ray “u” in that order, thereby resulting in a ghost image appearing on the screen 102. The ray “u”, which has passed through three boundary surfaces following the reflected ray “d2”, attenuates only to about 88% even if 4% is lost by reflection at every boundary surface. Accordingly, the ray “u” still has a quantity of light equivalent to 3.5% (=4%×88%) of the incident ray “b2” and, therefore, deteriorates video image quality.
In cases where a mirror that is approximately parallel to a screen is employed as a plane mirror for folding an optical path so as to reduce the depth of the apparatus, the size in the depth direction becomes about half, but the size in the height direction becomes much larger than the height of the screen. In particular, if the minimum value of an incident angle is made larger for the purpose of overcoming the above-mentioned problem arising when the incident angle is small, the height of the apparatus inevitably becomes very large. Therefore, in the case of a screen using the total-reflection-prism-type Fresnel lens, it is preferable to use a plane mirror that is approximately perpendicular to the screen. If an angle between the plane mirror and the screen is made slightly smaller than a right angle, the depth of the apparatus can be made minimum. However, in this instance, another problem arises in that, among reflected rays from the prism surface of the screen, a ray which has not enter the adjoining prism reflects from the plane mirror and then returns again to the screen, thereby causing a ghost image.
Such a phenomenon is described with reference to FIG. 13. FIG. 13 is a schematic sectional side view showing optical paths inside a rear-projection-type video apparatus 101. Among rays projected from a light source unit 104 by a projection optical system 141, a ray “a” corresponding to the center of an image plane is reflected from a plane mirror 103 and becomes a ray “b”. The ray “b” is incident on a total-reflection Fresnel screen 102 and is then bent as a ray “c”. The ray “c” emerges from the total-reflection Fresnel screen 102. Here, a certain fraction of reflected light from an entrance surface of the Fresnel screen 102 (i.e., the entrance surface 181 of the prism PR101) enters an adjoining prism again, thereby becoming a ghost image, as mentioned above. The remainder, which has not entered the adjoining prism again, advances as a ray “d”. The ray “d” is reflected from the plane mirror 103 and becomes a ray “e”. The ray “e” is disadvantageously incident on a position different from the original position on the screen 102. The ray “e” is then bent as a ray “f” by the total-reflection Fresnel action. The ray “f” emerges from the screen 102, which will be viewed as a ghost image. Likewise, a ray “g” that proceeds to the bottom side of the screen 102 advances as a ray “h”, a ray “j”, a ray “k” in that order and becomes a ray “l”, which will be also viewed as a ghost image. Furthermore, the total-reflection Fresnel screen 102 functions as a kind of Fresnel concave mirror when reflecting the ray “b” in the direction of the ray “d” and, therefore, has a light collecting function. Accordingly, a ghost image having very high brightness will be viewed.